What is fastest to do when filling up a Map?

cmpSpeed.scala

    def generateRandom(length: Int): String = scala.util.Random.alphanumeric.take(length).mkString("")

    def timeInMs[R](block: => R): (Long, R) = {
      val t0 = System.currentTimeMillis()
      val result = block // call-by-name
      val t1 = System.currentTimeMillis()
      ((t1 - t0), result)
    }


    val howManyFields = 10
    val fieldNames = (1 to howManyFields).map(_ => generateRandom(10)).toArray
    val howManyRuns = 100000

    // with a map:
    val (time1, m1) =
      timeInMs {
        (1 to howManyRuns).foldLeft(Map.empty[String, List[String]]) { (m, _) =>
          val fieldName = fieldNames(scala.util.Random.nextInt(howManyFields))
          val newValue = generateRandom(10)
          m + (fieldName -> (m.getOrElse(fieldName, List.empty) ++ List(newValue)))
        }
      }

    // with a list + groupBy:
    val (time2, m2) = {
        timeInMs {
          val x =
            (1 to howManyRuns).foldLeft(List.empty[(String, String)]) { (m, _) =>
              val fieldName = fieldNames(scala.util.Random.nextInt(howManyFields))
              val newValue = generateRandom(10)
              m ++ List((fieldName, newValue))
            }
          x.groupBy(r => r._1).map{case (k,v) => k -> v.map(_._2)}
        }
    }